Stainless steel

ABSTRACT

A LOW COST TERRITIC STAINLESS STEEL WITH EXCEPTIONALLY GOOD CORROSION RESISTANT PROPERTIES HAVING THE FOLLOWING COMPOSITION:   CARBON -0.1% MAXIMUM MANGANESE 2.0% MAXIMUM. SILICON 0.5% MAXIMUM. CHROMIUM 6-12%. ALUMINUM 2-7%. COPPER 0.2-3.0%. MOLYBDENUM UP TO 2.0%. IRON BALANCE.   WHEREIN CHROMIUM PLUS ALUMINUM IS AT LEAST ABOUT 13.0% AND COPPER PLUS MOLYBDENUM IS AT LEAST 0.5%.

United States Patent Ofice 3,690,870 STAINLESS STEEL Eugene Williams,Pittsburgh, Pa., assignor to United States Steel Corporation No Drawing.Filed Aug. 26, 1970, Ser. No. 67,293 Int. Cl. C22'c 37/10, 39/54 US. Cl.75-124 2 Claims ABSTRACT OF THE DISCLOSURE A low cost ferritic stainlesssteel with exceptionally good corrosion resistant properties having thefollowing composition:

Carbon 0.1% maximum. Manganese 2.0% maximum. Silicon 0.5% maximum.Chromium 6l2%. Aluminum 2-7%.

Copper 0.2-3.0%. Molybdenum up to 2.0%.

Iron balance.

wherein chromium plus aluminum is at least about 13.0% and copper plusmolybdenum is at least 0.5%.

BACKGROUND OF THE INVENTION This invention relates generally tostainless steel, and more specifically, to a new, low cost, 6 to 12percent chromium stainless steel having corrosion resistance equal to orbetter than the conventional 17 percent chromium stainless steels.

The superior corrosion resistance of commercial stainless steels isprimarily due to the addition of chromium to the alloy in amounts whichvary from 4 to 30 percent. Since the corrosion resistance of stainlesssteel is more or less a direct function of the chromium content, it iswell accepted that those stainless steels having large amounts ofchromium are usually superior in corrosion resistance to those havinglesser amounts.

The three most common stainless steels in commercial use today are theAISI (American Iron and Steel Institute) Types 410, 430 and 304 whichcontain about 12, 17 and 18 percent chromium respectively. Of thesethree most common stainless steels, AISI Type 304, having the greatestamount of chromium plus about 8% nickel, is the best for conventionalcorrosion resistance applications.

In addition to being one of the major constituents for corrosionresistance, chromium is also the principal cost element in mostcommercial stainless steels. Therefore, AISI Type 304 stainless steel,in addition to being superior in corrosion resistance, is also moreexpensive than AISI Types 410 and 430 or most other lower chromiumstainless steel.

Because of the relatively high cost of chromium, there are continuingefforts to find replacements for at least a portion of chromium by othercheaper elements that would reduce production costs yet maintain areasonably high degree of corrosion resistance.

In my co-pending joint patent application, Ser. No. 829,110 now Pat. No.3,594,156, there is disclosed one such stainless steel having only 6 to12 percent chromium, and yet having corrosion resistance characteristicsequal to or greater than AISI Type 304 stainless steel. In that steelsmall, relatively controlled amounts of aluminum, copper, molybdenum andcolumbium are added to the steel to more than compensate for the reducedchromium content.

SUMMARY OF THE INVENTION This invention is predicated upon mydevelopment of a new statinless steel wherein a portion of the chromium3,690,870 Patented Sept. 12, 1972 content is replaced by controlledamounts of aluminum and copper and with optional additions ofmolybdenum. No columbium is added to this steel. This lower coststainless steel contains chromium in amounts of from 6 to 12 percent,and yet has corrosion resistance characteristics equal to or greaterthan AISI Type 430 stainless steel. Although the steel of this inventionis not quite as corrosion resistant as is the alloy containingcolumbium, i.e., it being comparable to AISI Type 430 rather than Type304, it is substantially cheaper without additions of columbium, andfurther, molybdenum can be completely eliminated where economy is themajor consideration.

Accordingly, it is an object of this invention to provide a new low coststainless having exceptionally good corrosion resistance and containingfrom 6 to 12 percent chromium and lesser controlled amounts of aluminumand copper with optional additions of molybdenum.

It is another object of this invention to provide a new low coststatinless steel having from 6 to 12 percent chromium and yet havingcorrosion resistance characteristics equal to or greater than the 17percent chromium stainless steels.

It is a further object of this invention to provide a new stainlesssteel having corrosion resistance characteristics equal to or greaterthan AISI Type 430 stainless steel and yet utilizing lesser amounts ofchromium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although aluminum, molybdenum,and copper are known to impart some small degree of corrosion resistanceto steel, I have found that there are substantial beneficialinteractions between certain combinations of these elements which evenfurther contribute to corrosion resistance. Even relatively smallcombined amounts of chromium and aluminum, or copper, with or withoutmolybdenum will render exceptional corrosion resistant characteristicsto a steel, I have further found that a combination of these beneficialinteractions can be used as a substitute for a large portion of thechromium in stainless steels without an adverse effect on corrosionresistance. These substituted elements being cheaper in cost and beingused in comparatively smaller amounts have the effect of making a highquality stainless steel cheaper to produce and yet maintainingexceptional corrosion resistance characteristics. A stainless steel inaccordance with this invention therefore may have corrosion resistancecharacteristics equal to or better than AISI Type 430 statinless steelwhile containing about half, or even less than half as much chromium andlow cost quantities of aluminum and copper, and molybdenum if desired tooptimize corrosion resistance.

The statinless steel of this invention is ferritic and has the followinggeneral composition:

Carbon 0.1% maximum. Manganese 2.0% maximum. Silicon 0.5% maximum.Chromium 6 to 12. Aluminum 2 to 7.

Copper 0.2 to 3.0%. Molybdenum 0.0 to 2.0%.

The balance of the steel should of course be iron (i.e., iron with otherusual steelmaking impurities). The above composition range is furtherlimited by the facts that the combination of chromium plus aluminum mustbe at least 13 percent or very near thereto and that the copper orcombination of copper plus molybdenum must be at least 0.5 percent.

Although as little as 2 percent aluminum will be beneficial to thecorrosion resistance properties of steels having the higher chromiumcontents, optimum results are obtained only if the aluminum ismaintained within the range of 5 to 7 percent at ideal chromium contentsof 6 to percent. Chromium contents toward the higher end of the rangewill of course provide slightly better corrosion resistance. Since theimproved corrosion resistance towards the extreme higher end of therange is quite slight, however, only about 8 percent chromium or from 6to 10 percent is preferred for most applications as a practical goodbalance between corrosion resistance and economy. Since the foremostessential requirement is that chromium plus aluminum equal at leastabout 13 percent, a provision for 8 percent chromium will make itessential that at least about 5 percent aluminum be added.

Although molybdenum may be completely eliminated, copper, on the otherhand, must be present in an amount 01 at least 0.5 percent, or at least0.2 percent when the steel contains 0.3 percent molybdenum or more.Ideally, copper is maintained between 0.7 and 1.0 percent for opnmumeffect on corrosion resistance, with or without the presence ofmolybdenum. Additions of molybdenum should be eliminated when economy isthe major consideration, but for optimum corrosion resistance,molybdenum in concentrations of at least about 1.0 percent is preferred.Increasing the molybdenum and/or copper contents above the preferred 1.0percent each will provide only slight improvement in corrosionresistance up to about 2.0 percent each, or up to about 3.0 percentcopper proper solution, the stainless metals inherently passivatewithout the applied force. The driving force for the electrochemicalreaction is provided by the currents resulting from local variations inpotential at the metal surface. The inherent ability to form aprotective film (passivate) requires that this reaction have athermodynamic and kinetic tendency greater than any other electrodereaction that might possibly take place.

A metal is passive if, on increasing the electrode potential toward morenoble values, the rate of anodic dissolution in a given environmentunder steady-state conditions becomes less than the rate at same lessnoble potential. The tendency for passivation of stainless steels is afunction of both a critical current density and a critical potential.Therefore, by measuring these parameters and comparing them with thosevalues for commercially available stainless steels, the passivationtendency, and hence the corrosion resistance characteristics, for anyexperimental heat can be readily evaluated. A strong tendency topassivate is manifested by an active passivation potential and a lowcritical current density.

Table I below contrasts the critical current density and chemistry ofAISI Type 430 stainless steel with a representative sampling of theexperimental heats, most of which had compositions in accordance withthis invention. AISI Type 430 stainless steel was shown to have acritical current density of 7.2 ma./cm.

TABLE I Chemical composition, by weight Critical Example current, No. 0Mn P S St Cr Al Cu Mo maJam 2 AISI 430.-.. 0. 610 0.65 0. 020 0. 0070.52 17. 10 0. 01 0.05 0. 05 7. 2 1 0. 52 0. 005 0. 003 0. 03 8. 49 4.93 1. 03 0. 018 5. 5 0. 51 0. 017 0. 005 0. 2 8.95 3. 08 0. 62 7. 2

1 Not determined residual amounts.

when molybdenum is absent. Beyond these limits no apparent improvementis obtained.

Accordingly, the preferred composition of this stainless steel, foroptimum corrosion resistance at a minimum cost, would be about 8 percentchromium, 5 percent aluminum and 1 percent each of copper andmolybdenum.

The effect of copper on corrosion resistance properties is actuallydetrimental at chromium contents below 6 percent, but of coursebeneficial at chromium levels above 6 percent. Therefore, the minimumchromium content of 6 percent is somewhat critical to avoid the adverseeffects of copper.

Nickel, titanium and silicon additions were considered for this alloy,but neither showed any beneficial effect. Nickel and titanium areneither beneficial nor detrimental. Silicon, on the other hand, not onlyhas a detrimental effect on corrosion resistance, but further adverselyaffects the metallurgical quality of the steel. Therefore, siliconshould be kept at residual impurity amounts of no more than 0.5 percent.

In order to quantitatively evaluate the stainless steels of thisinvention, over 70 heats of steel having varying compositions weretested for actual corrosion resistance and potentiodynamic polarization.The potentiodynamic polarization test is based upon the fact thatstainless steels achieve their high resistance to corrosion by theirability to spontaneously passivate (i.e., form a protective oxide film)in the corrosive environment. Although active metals such as iron can bemade to passivate electrolytically by the application of an electricalcurrent or potential in a In Table I above, it is readily apparent thatthe stainless steels having compositions in acordance with thisinvention do have equal or lower critical current densities than theAISI Type 430 stainless steel, and hence, equal or greater corrosionresistance tendencies.

Upon careful examination of the specific examples shown in the aboveTable I, it should be noted that the last three examples hadcompositions outside the critical ranges taught above. Examples 13contained insufficieut aluminum at 1.94 percent, Example 14 containedinsufficient chromium plus aluminum at 12.05 percent, and Example 15contained insuflicient copper at 0.31 percent. It should be furthernoted that each of these examples displayed higher critical currentdensities than the standard, AISI 430. It may be further noted thatExamples 2, 4 and 10 contained very slightly less than 13 percentcombined chromium plus aluminum, namely 12.93 percent, 12.98 percent and12.93 percent respectively. These examples were, however, sufficientlyclose to the 13 percent limit that they did possess the desired lowercritical current densities. Examples 13 and 14 containing combinedaluminum and chromium levels of 12.24 percent and 12.05 percentrespectively were insufficient, and hence, substantially inferior to thestandard AISI 430 steel.

The above potentiodynamic polarization tests were verified by exposingsamples of my stainless steel along with samples of AISI Type 430stainless steel to the corrosive atmosphere of a coke making plant.After seven months exposure, my stainless steels were substantially lesscorroded than the AISI Type 430 samples. In addition, the test heatswere given chemical corrosion tests in sulfuric acid and sodiumchloride. Table II below shows typical results contrasting twoexperimental heats with AISI Type 430. The first experimental heat wasExample 12 also shown in Table I above. The second experimental heat,Example 16, is a heat (not shown in Table I) having a compositionoutside the scope of this invention.

bined chromium plus aluminum content being at least about 13.0 percent,and the combined copper plus molybdenum content being at least 0.5percent.

2. A low chromium stainless steel according to claim 1 having about 8percent chromium, 4 to 7 percent aluminum, and about 1 percent each ofcopper and molybdenum.

TABLE II Atmosphere corrosion tests,

6 months Wet chemical corrosion tests 5% H1804 at 3%% NaCl at Majoralloying elements Weight loss, 1;. room temp., room temp., 47 hrs.,weight 47 hrs., weight Example N0. Heat No. Cr Al Cu Mo Test #1 Test #2Average loss, gm. loss, gm.

Standard AISI 430-- 17.00 0.06 0. 14 0. 3031 0.3204 0.3117 5.4365 0.009512 221047 8. 98 4. 86 1. 03 0. 2843 0. 3233 0. 3038 3. 2275 0.0012 16221049 8. 05 3. 97 0. 42 0. 22 0. 4795 0. 4860 0. 4827 6. 550 0. 0090 1See footnote at end of table I.

Examination of Table II will show that the steel in accordance with thisinvention (Example 12) was indeed superior in corrosion resistance toAISI Type 430 steel. Example 16, which contained an insuflicient amountof copper and an insufliicient amount of chromium plus aluminum (12.02percent) was inferior to AISI Type 430 steel.

I claim:

1. A low chromium ferritic stainless steel consisting essentially of upto 0.10 percent carbon, up to 2.0 percent manganese, 6 to 12 percentchromium, 2 to 7 percent aluminum, 0.2 to 3.0 percent copper, up to 2.0%molybdenum, and the balance substantially iron with the com- ReferencesCited UNITED STATES PATENTS 'HYLAND BIZOT, Primary Examiner US. Cl. X.R.75l25

